pg_buffercache: Add pg_buffercache_os_pages

pg_buffercache: Add pg_buffercache_os_pages

ba2a3c2302f has added a way to check if a buffer is spread across
multiple pages with some NUMA information, via a new view
pg_buffercache_numa that depends on pg_buffercache_numa_pages(), a SQL
function.  These can only be queried when support for libnuma exists,
generating an error if not.

However, it can be useful to know how shared buffers and OS pages map
when NUMA is not supported or not available.  This commit expands the
capabilities around pg_buffercache_numa:
- pg_buffercache_numa_pages() is refactored as an internal function,
able to optionally process NUMA.  Its SQL definition prior to this
commit is still around to ensure backward-compatibility with v1.6.
- A SQL function called pg_buffercache_os_pages() is added, able to work
with or without NUMA.
- The view pg_buffercache_numa is redefined to use
pg_buffercache_os_pages().
- A new view is added, called pg_buffercache_os_pages.  This ignores
NUMA for its result processing, for a better efficiency.

The implementation is done so as there is no code duplication between
the NUMA and non-NUMA views/functions, relying on one internal function
that does the job for all of them.  The module is bumped to v1.7.

Author: Bertrand Drouvot <[email protected]>
Reviewed-by: Mircea Cadariu <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Discussion: https://postgr.es/m/Z/[email protected]